Talks

A key subroutine in quantum computing, especially in quantum simulation, is to prepare thermal states or ground states of Hamiltonians. Today, I will talk about a new family of quantum algorithms for this task. Physically, our algorithms distill the essence of system-bath interaction by simulating an effective Lindbladian; computationally, our algorithms are quantum analogs of classical Markov chain Monte Carlo sampling. Given the ubiquity of thermodynamics and the triumph of classical Monte Carlo methods, we anticipate that quantum thermal state preparation will become indispensable in quantum computing. 

Joint work with Michael J. Kastoryano, Fernando G.S.L. Brandão, and András Gilyén. https://arxiv.org/abs/2303.18224 

Zoom Link: https://pitp.zoom.us/j/91641127738?pwd=cExPM3Bvd3BaYnJYS0U0UlBiVTJ0QT09

Holography has profoundly transformed our understanding of quantum gravity in spacetimes with asymptotic negative curvature. Its implications for cosmology are equally profound, suggesting that time is emergent and that our universe has a dual description in terms of a three-dimensional quantum field theory. This talk will outline key features of holographic cosmology, from the perspective it offers for the cosmic singularity to the strategies it presents for computing cosmological observables. Recent results for the de Sitter wavefunction will be discussed and their interpretation in the language of three-dimensional conformal field theory.

The classical spacetime manifold of general relativity disappears in quantum gravity, with different research programs suggesting a variety of alternatives in its place. As an illustration of how philosophers might contribute to an interdisciplinary project in quantum gravity, I will give an overview of recent philosophical debates regarding how classical spacetime "emerges."  I will criticize some philosophers as granting too much weight to the intuition that a coherent physical theory must describe objects as located in space and time.  I will further argue, based in part on historical episodes, that an account of emergence needs to recover the structural features of classical GR responsible for its empirical success.  This is more demanding than it might at first appear, although the details of recovery will differ significantly among different approaches to quantum gravity.

Zoom link:  https://pitp.zoom.us/j/98331676824?pwd=VTNOakMxWWUzT2ZFZFYwYzBRdWxBUT09

If relativistic gravitation has a quantum description, it must be meaningful to consider a spacetime metric in a genuine quantum superposition. Here I present a new operational framework for studying “superpositions of spacetimes” via model particle detectors. After presenting the general approach, I show how it can be applied to describe a spacetime generated by a BTZ black hole in a superposition of masses and how such detectors would respond. The detector exhibits signatures of quantum-gravitational effects reminiscent of Bekenstein’s seminal conjecture concerning the quantized mass spectrum of black holes in quantum gravity. I provide further remarks in distinguishing spacetime superpositions that are genuinely quantum-gravitational, notably with reference to recent proposals to test gravitationally-induced entanglement, and those in which a putative superposition can be re-expressed in terms of dynamics on a single, fixed spacetime background.

The constrained Hamiltonian formalism is the basis for canonical quantization techniques. However, there are disagreements surrounding the notion of a gauge transformation in such a formalism. The standard definition of a gauge transformation in the constrained Hamiltonian formalism traces back to Dirac: a gauge transformation is a transformation generated by an arbitrary combination of first-class constraints. On the basis of this definition, Dirac argued that one should extend the form of the Hamiltonian in order to include all of the gauge freedom. However, Pitts (2014) argues that in some cases, a first-class constraint does not generate a gauge transformation, but rather "a bad physical change". Similarly, Pons (2005) argues that Dirac's analysis of gauge transformations is "incomplete" and does not provide an account of the symmetries between solutions. Both authors conclude that extending the Hamiltonian in the way suggested by Dirac is unmotivated. If correct, these arguments could have implications for other issues in the foundations of the constrained Hamiltonian formalism, including the Problem of Time. In this talk, I use a geometric formulation of the constrained Hamiltonian formalism to show that one can motivate the extension to the Hamiltonian independently from consideration of the gauge transformations, and I argue that this supports the standard definition of a gauge transformation without falling prey to the criticisms of Pitts (2014) and Pons (2005). Therefore, in order to maintain that first-class constraints do not generate gauge transformations, one must reject the claim that the constrained Hamiltonian formalism is fully described by the geometric picture; I suggest two avenues for doing so.

The success of the AdS/CFT correspondence motivates a holographic approach for spacetime beyond AdS, including our own universe. One possible method involves using an asymptotically-AdS holography and introducing a finite radial cutoff by inserting an End-of-World (EoW) brane. However, previous studies have shown that this leads to nonlocal effects on the boundary and violates entanglement sub-additivity. In this work, we address these issues by examining a two-particle scattering process through the lens of holographic quantum tasks. Our findings suggest that connectedness of entanglement wedge does indeed require nonlocal domain of dependence, but that violation of sub-additivity can be avoided. We also discuss an important question that arises from our results, namely whether the non-locality on the EoW brane is real or apparent. We argue that it is the latter. This talk is based on ongoing work with Takato Mori.

For the last ten years I have been documenting various scenarios for the early universe in a YouTube series called ‘Before the Big Bang”. Having interviewed many of the leader figures of the field including Stephen Hawking, Roger Penrose, Alan Guth (and hosted debates between them), this will be a broad survey of inflation, it’s suggested prequels and alternatives. I shall highlight the strengths and weaknesses of the various proposals and give an inside track of the claims and counter claims in attempts to move beyond the standard Big Bang model.